Replace Default trait bound with a zero function on Summary/Dimension (#17975)

This lets us provide a context when constructing the zero value. We need
it so we can require anchors to be associated with a buffer id, which
we're doing as part of simplifying the multibuffer API.

Release Notes:

- N/A

Co-authored-by: Nathan <nathan@zed.dev>

crates/language/src/diagnostic_set.rs

@@ -15,7 +15,7 @@ use text::{Anchor, FromAnchor, PointUtf16, ToOffset};
 /// The diagnostics are stored in a [`SumTree`], which allows this struct
 /// to be cheaply copied, and allows for efficient retrieval of the
 /// diagnostics that intersect a given range of the buffer.
-#[derive(Clone, Debug, Default)]
+#[derive(Clone, Debug)]
 pub struct DiagnosticSet {
     diagnostics: SumTree<DiagnosticEntry<Anchor>>,
 }
@@ -135,7 +135,7 @@ impl DiagnosticSet {
     {
         let end_bias = if inclusive { Bias::Right } else { Bias::Left };
         let range = buffer.anchor_before(range.start)..buffer.anchor_at(range.end, end_bias);
-        let mut cursor = self.diagnostics.filter::<_, ()>({
+        let mut cursor = self.diagnostics.filter::<_, ()>(buffer, {
             move |summary: &Summary| {
                 let start_cmp = range.start.cmp(&summary.max_end, buffer);
                 let end_cmp = range.end.cmp(&summary.min_start, buffer);
@@ -261,6 +261,10 @@ impl Default for Summary {
 impl sum_tree::Summary for Summary {
     type Context = text::BufferSnapshot;
 
+    fn zero(_cx: &Self::Context) -> Self {
+        Default::default()
+    }
+
     fn add_summary(&mut self, other: &Self, buffer: &Self::Context) {
         if other.min_start.cmp(&self.min_start, buffer).is_lt() {
             self.min_start = other.min_start;